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Malagasy/Indo-Australo-Malesian Phytogeographic Connections

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Malagasy/Indo-Australo-Malesian Phytogeographic Connections Biogéographie deMadagascar, 1996 : 73-83 MALAGASYLENDO-AUSTRALO-MALESIANPHYTOGEOGRAPH" CONNECTIONS George E. SCHATZ Missouri Botanical Garden, PO. Box 299, St. Louis, Missouri, 63166-0299, U.S.A. USTRACT.- Despite the continuous close proximityof Madagascar to Africa sinceits separation from the continent ca. 165 MXA, the Malagasy flora exhibits a remarkably high affinity with the Indo- autralo-malesian florasfar to the east. Such phytogeographic connectionsare especially prevalent among easternhumid forest taxa, and representboth ancient vicariance that hasresulted in relictual (Cretaceous) Gondwanan disjunctions, as well as continuous dispersal events across the Indian Ocean. Three major patterns of dispersaVvicariance modality can be identified: 1) Cretaceous dispersal to Madagascar with ensuing distributions from India (andor South Africa) across Antarctica to South America and Australo-E. Malesia during the timeof the initial radiation of the angiosperms; 2) Eocene- Oligocene (and continuing to the present) dispersal to Madagascar (and Africa) from Laurasia and W. Malesia viaIndia @re- and post-collision with Asia) alongt( Lemurian Stepping-stones)) in the western IndianOcean; and 3) continuous(and recent) long distance dispersal (LDD) to Madagascar as a function of the prevailing easterlywinds and Indian Ocean currents. KEY-W0RDS.- Madagascar, Indo-Australo-Malesian, Phytogeography, Vicariance, Dispersal RESUME.- Malgré la proximité de Madagascar et de l'Afrique depuis leur séparation,il y a quelques 165 millions d'années, la flore malgache montre unefinité remarquable avec celles dela région Indo- australo-malésienne très éloignée vers l'est. Ces relationsphytogéographiques sont particulièrement fréquentes dans les taxons des forêts humides de l'est de Madagascar; elles représentent d'unepart une vicariance ancienne qui a entraîné des disjonctions reliques gondwaniennes (Crétacé) et d'autre part une série continue de dispersionà travers l'Océan Indien. Trois modèles principaux de dispersionlvicariance à Madagascar peuvent être distingués : 1) Au Crétacé une dispersion qui a entraîné des types de répartition qui s'étendent de l'Inde (et/ou l'Afrique du Sud) jusqu'à l'Amérique du Sud et le région australo-est-malesienne àtravers l'Antarctique,pendant la périodeinitiale de diversification des angiospermes ; 2) Pendant 1'Eocène-Oligocène une dispersion encore actuelle et également africaine, de taxons Laurasiens et ouest Malésiens via l'Inde avant et après son entré en collision avec 1'Asia par l'intermédiaire d'îles (t< Lemurian Stepping-Stones B) situées dans l'ouest de l'Océan Indien ; 3) Une dispersion à longuedistance continue et plus récente due aux vents et auxcourrants océaniques dominants de l'est. MOTS-CLES.- Madagascar,Région Indo-australo-malésienne, Phytogéographie,Vicariance, Dispersion INTRODUCTION As a former piece of the African continent that has remained in close proximity throughout its evolutionary history, Madagascarwould reasonably be expected to In: W.R. LOURENçO (éd.) Editions de I'ORSTOM, Paris 74 G.E. SCHATZ possessbiotaa whose closestaflinities are African.Recognition therefore of phytogeographic connections with floras far to the east has attracted the interest of botanists since the classic work of PERRTER DE LA BATHIE (1936). Subsequent workers have adopted and refinedthe characterization of floristic af€inities by percentage generic overlapwithin arbitrarily delimited geographic units (tt elements B), amongwhich <t Asian D, tt oriental D, <t paleotropical )),and <t austral )) have been utilized to reflect the easternailinities (HWERT, 1959;KOECHLIN et al., 1972; DEJARDIN et al., 1973). However, as LEROY (1978)correctly pointed out, such analyses of relationship depend upon taxonomic circumscription of genera rather than phylogenetic hypotheses,and fail to addressprocess in historical biogeography. The distinction made byAUBREVILLE (1976) between . <t Australo-Papoue )) as Gondwananelements and tt Asiatico- Malesienne )) as Laurasian elements set the stage for a more explicit discussion of the historical patterns leading to modern-day relationships betweenthe floras of Madagascar andIndo-australo-malesia. In that context,utilizing data fiom recent phylogenetic, paleobotanical, and paleogeographic studies, 1 would like to illuminate fbrther what 1 believe to be the three major modes of dispersdvicariance between Madagascar and lands to the east: 1) Cretaceous Gondwanan Relicts; 2) Eocene-Oligocene <t Lemurian Stepping-Stones D; and 3) Long Distance Dispersa1 (LDD). CRETACEOUS GONDWANAN RELICTS Paleomagnetic data from the westernIndian Ocean has firmly established the paleogeographic history of Madagascar (RABINowITZ et al., 1983). Figure 1 depicts a schematic representation of Gondwanan land masses fiom the time Madagascar arrived at its current position 121 MYA until the separation of Greater India fiom Madagascar 88 MYA(STOREY et al., 1995), by which time the break-up of virtually all of the individualGondwanan land masses had begun. Throughout the periodrifting was occurring both between AfricdSouth America and Greater Indidhtarctica, although eachpair remained within close proximitythrough the end of the Cretaceous/early Tertiasy. By 80 MYA, direct land routes between New Zealand and New Caledonia and AntarcticdAustralia had been severed (MILDENHALL, 1980); very slow rifiing between AustraliaNew Guinea and Antarctica had also begun, but New Guinea did not emerge above sea level until the late Eocene at which time Gondwanan elements first entered (AXELROD& RASEN, 1982). A dispersal pathway through Antarctica to South America remained possible untilthe close of the Eocene (RAS?EN, 1979). Thetime period spanned in Figure 1 isalso contemporaneous with the initial radiation of the angiosperms. Insofar as Madagascar began to separate from fica during the late Jurassic (165 MYA), it is probable that angiosperms were not present on the island during mostof its slow raRing to its current position. Paleobotanicaldata from Madagascar at this critical time period are scant. Palynomorphs fiom mid-Cretaceous (middle Albian to late Cenomanian) samples near Ahtsiranana belong to the Gondwana microfloral Province, dominated by bisaccate gymnosperms and ferns, but are lacking angiosperms (HERNGREEN et al., 1982).Another Cenomanian assemblage fiom Ankanotra is dominated by the conifer Classopollis; however, six different angiosperm pollen types are present. Despite the lack of late Cretaceous data, it is reasonable to assume that by the close of the Mesozoic, numerous angiosperm lines had established themselves in Madagascar via either direct land routes through India from the south, or by short distance dispersalacross water gaps. PHYTOGEOGWHIC CONNECTIONS 75 Whatneobotanical evidence therefore (aswell as paleobotanical data fiom elsewhere in Gondwana) points to the presence of Cretaceous Gondwanan relicts in Madagascar? The break-up of Gondwana by the end of the Cretaceous would have resulted in vicariance in basal angiosperm lineages, and thus, relationships today would be manifest at higher taxonomic levels(VAN STEEMS, 1984). Modern distributions would beessentially austral and/orcool mesic (pre)montane: South America,extendmg northward along the Andes into Central America; Mca, especially cooler upland East Afi-ica (with survival into the Miocene and then extinction in South Africa (COETZEE& MULLER, 1984)); Madagascar; and Australo-malesia east of Wallace's line,i.e., Australia, New Caledonia,New Guinea, and New Zealand. The possiblity of << Noah's Ark )) rafting on India, and therefore the presence of relicts on the granitic islands of the Seychelles and IndidSri Lanka, as well as then subsequent dispersal into W. Malesia must also be considered (a scenariorejected by RAw & AXELROD,1974). Mid to late Cretaceous/earlyTertiary floras from Antarctica, Australia and New Zealand (BIRKENMAJER & ZASTAWNIAK, 1989; BURGER, 1993; DETTMAN, 1989; DETTMAN& JARZEN, 1990; MILDENHALL, 1980)confirm the notion of a subhumidmesothermal climate across Antarctica,which supported a broad-leaved forest withnumerous angiospermpollen types. Spacepermits the discussionof only a fewexamples to illustrate the distribution patterns above: Fig. 1. Schematic representation of Gondwanan land masses during initial Cretaceous radiation of the angiosperms,between the time of arriva1 of'Madagascar atits present position(121 MA) andthe time of separation of GreaterIndia from Madagascar (88 MYA). Throughout the time period, rifting is occurring between both AfiicdSouth America and Greater IndidAntarctica, although each pair remains within close proximity at the end of the period. India is also shown (broken border) after collision with Asia (45 MA), at which point a direct link with W. Malesiais established. Abreviations: MADAG = Madagascar; N.C. = New Caledonia; N.G. = New Guinea (submerged until late Eocene); N.Z. = New Zealand. Proteaceae: The studies of JOHNSONand BRIGG~(198 1) have revealed two independent relict lines of Proteaceae in .Madagascar. Isolated dioecious Dilobeia (2 76 G.E. SCHATZ spp.), the solemember of subtribeDilobeiinae appears to be sister to the subtribe Cenarrheninaeconsisting oftwo monotypicgenera on Tasmania (Agastachys and Cenarrhenes), two genera endemic to New Caledonia (Beaupreopsis (1 sp.) and Beauprea (12 spp.; with fossil distribution in Australia and New Zealand)), and a more xeric taxon Symphionema (2 spp.) in N.S. Wales. From a base number of x = 14 for subfamily Proteoideae, chromosomalevolution
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